The present invention relates to a rear wheel steering apparatus for an automobile for turning rear wheels together with front wheels upon operation of a steering wheel and, more particularly, to a rear wheel steering apparatus for an automobile capable of changing a ratio of a rear wheel turning angle to a front wheel turning angle (to be referred to simply as a rear wheel turning angle ratio hereinafter) in accordance with a vehicle speed.
A known four-wheel steering apparatus comprising a rear wheel steering apparatus for an automobile (or vehicle) turns front and rear wheels in accordance with an operation of a steering wheel, and changes the rear wheel turning angle ratio based on a predetermined rear wheel turning angle ratio characteristic in accordance with a vehicle speed, as disclosed in Japanese Patent Publication No. 60-44185. In this case, the rear wheel turning angle ratio characteristic is set as follows. More specifically, the front and rear wheels are turned in an opposite phase state at a low vehicle speed lower than a predetermined vehicle speed (e.g., 35 km/h) to improve a turning radius of a vehicle. At the predetermined vehicle speed, the rear wheels are not turned to set a so-called 2WS mode. At a high vehicle speed higher than the predetermined vehicle speed, the front and rear wheels are steered in an in-phase state to set a so-called 4WS mode, thereby improving travel stability of a vehicle (lane change stability).
In a conventional rear wheel steering apparatus of this type, when a vehicle travels along a curved road at a vehicle speed higher than the predetermined vehicle speed, since the front and rear wheels are turned in the in-phase state, a so-called high-speed cornering characteristic can be maintained well. During high-speed cornering, if a driver finds an obstacle ahead and quickly depresses a brake pedal, the vehicle is quickly decelerated, and the rear wheel turning angle ratio is quickly changed to the opposite phase state, accordingly. For this reason, in order to maintain a stable vehicle position during cornering and to assure stable travel, the front and rear wheels must firmly grip the road surface. Nevertheless, the degree of in-phase state is decreased, and finally, the rear wheels are turned in the opposite phase state. As a result, an inertial force acts in a vehicle body to generate a yaw rate, and accidental turning tendency of the vehicle is enhanced. Such a phenomenon appears as a so-called tuck-in phenomenon.
In particular, when the rear wheels are locked and slip by braking, the following problems are posed. That is, in this case, although the vehicle body travels at a certain speed, a vehicle speed sensor used for controlling the rear wheel turning angle ratio detects a wheel speed. For this reason, the output from the vehicle speed sensor immediately becomes zero upon locking. As a result, the rear wheel turning angle ratio is immediately changed to the maximum value of the opposite phase state according to the detection result from the vehicle speed sensor. In combination with a decrease in gripping force of tires upon locking, a very unstable travel state may occur.
For this reason, as disclosed in Japanese Utility Model Publication No. 62-5974, in a vehicle-speed sensitive rear wheel steering apparatus for a vehicle, when a quick deceleration of a vehicle is detected, the rear wheel turning angle ratio is set in an in-phase range to obtain a steering angle corresponding to the travel condition of the vehicle (an angle for increasing a slip angle), thereby preventing so-called tuck-in.
As disclosed in Japanese Patent Laid-Open (Kokai) No. 59-81275, in a vehicle-speed sensitive rear wheel steering apparatus for a vehicle, when a quick deceleration of a vehicle is detected, a change in rear wheel turning angle ratio is delayed to prevent the above-mentioned tuck-in phenomenon.
As a known means for detecting the quick deceleration of the vehicle, a vehicle speed sensor is used, and the quick deceleration is determined when the output from the vehicle speed sensor is quickly decreased. Alternatively, both a brake sensor and the vehicle speed sensor are used, and the quick deceleration is determined when the output from the vehicle speed sensor is quickly decreased after a brake pedal is depressed.
As disclosed in Japanese Patent Laid-Open (Kokai) No. 59-81274, upon quick deceleration, as alternative means of a technique for delaying a change in rear wheel turning angle ratio, a means for fixing the rear wheel turning angle ratio or a means for setting the rear wheel turning angle ratio to "0" to set the 2WS mode is known.
However, in these prior arts, after the quick deceleration of a vehicle is detected, the rear wheel turning angle ratio is controlled to prevent tuck-in. For this reason, the following problems are posed.
More specifically, when a quick deceleration of a vehicle is detected, a time of generation of the quick deceleration is very short, e.g., in the order of several hundreds of msec. Meanwhile, a sampling time of the vehicle speed sensor for controlling the rear wheel turning angle ratio is 131 msec at present. For this reason, in actual deceleration detection, only 2 to 3 detection results can only be obtained every 131 msec, and it is impossible to calculate the deceleration per unit time based on such a small amount of data so as to accurately determine whether or not the present deceleration falls in a quick deceleration range. In this manner, the conventional apparatus can only discriminate a deceleration with considerable inaccuracy.
When the rear wheel turning angle ratio is to be changed based on such an inaccurate quick deceleration discrimination result, a quick deceleration may be erroneously discriminated even if it is not so. If such erroneous discrimination is made, although the rear wheel turning angle ratio should be set in an opposite phase state as the vehicle speed is decreased, the in-phase state is maintained. Although the rear wheel turning angle ratio should be, for example, fixed to maintain a safe travel state when a vehicle is quickly decelerated in practice, the quick deceleration cannot be determined due to inaccuracy of the vehicle speed sensor, and the rear wheel turning angle ratio is not fixed but may be changed in an opposite phase state. Thus, a travel property of the vehicle is greatly impaired due to inaccuracy of the current vehicle speed sensor.
Since this vehicle speed sensor is conventionally of a contact type, so-called chattering often occurs. When chattering occurs in the vehicle speed sensor, a control mechanism determines a quick acceleration since an input signal is temporarily increased. Meanwhile, when the chattering terminates, the normal number of input signals is recovered. Therefore, the control mechanism determines a quick deceleration as a reaction of the preceding quick acceleration discrimination.
In this manner, when a quick deceleration is erroneously determined, the rear wheel turning angle ratio is fixed in the in-phase state although it need not be fixed, and the travel property of the vehicle is impaired.
In the vehicle-speed sensitive rear wheel steering apparatus using the existing vehicle speed sensor, a travel property may be impaired due to inaccuracy of the vehicle speed sensor, and since only the quick deceleration is taken into consideration, an unnecessary or erroneous fixing operation of a rear wheel turning angle ratio is performed, and a travel property is spoiled.
When a quick deceleration of a vehicle is detected or when both the vehicle speed sensor and the brake sensor are used, the following problems are pointed out in addition to the above-mentioned problems of the vehicle speed sensor.
When a vehicle travels along a road of a bad condition, if a wheel climbs over a projecting portion of a three-dimensional pattern on the road surface while a brake pedal is being depressed, the rotational speed of the wheel is instantaneously decreased, and the detection result of the vehicle speed sensor may fall in the quick deceleration range. In this case, if this travel state occurs during cornering, the rear wheel steering angle ratio is fixed in the in-phase state in the conventional apparatus. Therefore, even if a driver intends to perform cornering with good turnability in the opposite phase state, the vehicle cannot be turned well, and a travel property is considerably impaired.
In this manner, in the conventional apparatus, when the brake pedal is quickly depressed to avoid danger during high-speed cornering, since control for fixing the rear wheel turning angle ratio in the in-phase state is performed in consideration of only the quick deceleration in the vehicle speed sensor, an unnecessary or erroneous fixing operation of the rear wheel turning angle ratio is performed, and a travel property is spoiled.